Copper alloy cleaning process

ABSTRACT

A process for removing surface oxides from copper alloys. For example, these alloys which form, on annealing, A12O3 and related oxides are uniquely cleaned by a process which consists of a duplex treatment requiring a first immersion in a hot alkaline solution followed by a second immersion in a hot mineral acid solution.

United States Patent Ford et a].

[ Mar. 7, 1972 [54] COPPER ALLOY CLEANING PROCESS [73] Assignee: OlinMathieson Chemical Corporation [22] Filed: Jan. 6, 1969 [21] Appl. No.:789,417

[52] US. Cl ..l34/3, 134/27, 134/28,

[51] Int. Cl. ..C23g l/04, (124g l/ 10, C23g H20 [58] Field ofSearch..l34/2, 3, 26, 27, 28, 29, 30

[56] References Cited UNITED STATES PATENTS 947,067 III 910 Thompson..l34/28 2,125,458 8/1938 Ostermann ..1 34/3 2,284,743 6/1942 Kawecke eta1. 1 34/29 UX 2,318,559 5/1943 Percival ..l34/3 UX 2,395,694 2/ 1946Spence et al. ..,l34/2 UX 2,726,970 12/1955 Toth ..134/3 3,003,89610/1961 Kendall 134/3 3,121,026 2/1964 Beigay et al .134/2 1,049,05412/1912 Coombs ..27 X/ 1,859,734 5/1932 George ..134/2 X 2,671,7173/1954 Ferguson 1 34/2 Primary ExaminerMorris O. Wolk AssistantExaminerBarry S. Richman Attorney-Richard S. Strickler, Robert H.Bachman, Donald R. Motsko and Thomas P. ODay [57] ABSTRACT A process forremoving surface oxides from copper alloys. For example, these alloyswhich form, on annealing, A 1 0;, and related oxides are uniquelycleaned by a process which consists of a duplex treatment requiring afirst immersion in a hot alkaline solution followed by a secondimmersion in a hot mineral acid solution.

12 Claims, No Drawings COPPER ALLOY CLEANING PROCESS Many copper basealloys, especially those containing aluminum, form complex thermaloxides which are difficult to remove by conventional copper cleaningtechniques.

For example, a copper alloy containing about 66 percent copper, 1.55percent aluminum, 1.0 percent iron, balance essentially zinc, forms arefractory thermal oxide during annealing that cannot be reliablyremoved using ordinary cleaning procedure. The metal cannot bereproducibly soldered or electroplated because of the residual annealingoxide.

in addition to causing soldering and plating problems, excessiveresidual A1 can cause excessive die wear and failure during fabricationoperations.

It is the object of the present invention to achieve easy andreproducible removal of these films.

lt is another object of the present invention to remove those filmswithin the confines of existing copper-base alloy cleaning equipment.

Other objects and advantages will become apparent from the followingdescription.

In accordance with the present invention the foregoing objects arereadily obtained. The process of the present invention comprises:immersing the surface to be cleaned in an aqueous alkaline solutionhaving apH above 10 for at least two (2) seconds, with the solutionbeing at a temperature of from 100 F. to the boiling point; and thenimmersing the surface to be cleaned in an aqueous mineral acid solutionat a temperature of from 75 to 200 F. for from two (2) seconds tominutes, said solution having an acid equivalent to an H 50, solution of3 to 50 percent by volume. Preferably, a conventional water rinse or airwipe is performed between steps and after the final step.

The exact time and the exact bath temperature in each of these two stepsis determined by the type of annealing operation and the character ofthe resulting oxide including its thickness and constitution.

The process is suitable to both continuous cleaning of strip and tobatch cleaning of fabricated parts.

Following the described cleaning process, copper base alloys coated withcomplex oxides can be successfully soldered and electroplated becausethe residual oxide resulting from annealing has been removed.

The process of the present invention is effective for any copper bascalloy. The present process is especially effective with copper alloyscontaining as alloying additions: aluminum, from 0.5 to 12 percent; andsilicon, from 0.5 to 5 percent. The process of the present invention isparticularly useful with respect to copper alloys bearing: complexoxides; alumina:

spinel type oxides, such as copper aluminate and zinc aluminate; nickeloxides; etc.

As indicated hereinabove, the process of the present invention utilizesa duplex treatment requiring a first immersion in a hot alkalinesolution followed by a second immersion in a hot mineral acid solution.

Concerning the first alkaline solution, any alkaline solution may beused which has a pH above and preferably a pH from 1 1 to 14. Preferredsolutions which may be used depend, of course, upon the particular alloyand the results desired. ln general, caustic soda is the preferredalkaline solution, but others may be readily employed such a lithium orpotassium hydroxide, for example. The alkaline solution should bemaintained at a temperature of from 100 F. to the boiling point, andpreferably at a temperature of from 160 F. to the boiling point. Thecopper alloy to be cleaned should be immersed in the solution for atleast 2 seconds and preferably for from 5 seconds to 1 minute.Naturally, longer treatment times may be employed, but in general noadvantage is obtained thereby. Generally, the treatment time should beunder 10 minutes. Naturally, temperature and time are related and theexact treatment conditions are dependent upon the alloy used and theresults desired.

The second immersion in acid may employ any mineral acid solution havingan acid equivalent to an H 80 solution of3 to 50 percent by volume. Ingeneral, any mineral acid may be used with sulfuric acid beingpreferred. Others which may be used include nitric and hydrochloric, forexample. The preferred solution has an acid equivalent to an H SO,solution of from 5 to 20 percent by volume. The mineral acid solutionshould be maintained at a temperature of from 75 to 200 F. andpreferably at a temperature of from to 175 F. A treatment time of atleast 2 seconds should be employed and preferably from 5 seconds to 1minute. The mineral acid treatment should generallynot exceed 5 minutesas in-some cases there is obtained an undesirable blush color fortreatment times in excess of about 5 minutes. The alloy is, of course,effectively cleaned; however, the color is undesirable commercially.

Naturally, both the first and second solutions should be aqueoussolutions.

Preferably, a conventional water rinse or air wipe is preferred betweenthe steps and after the final step.

The present invention will be more readily apparent from a considerationof the following illustrative examples.

EXAMPLE I In this example, the following alloys were processed.

Copper, essentially balance The alloys were processed as shown in Table11, below, which shows bath concentration and residence times andtemperatures for air, bell, and strip annealing oxides for each alloy.

Air annealing was carried out as follows: The alloy sheet was brought toa temperature of about l,l00 F. in several minutes and held at thattemperature for 2 hours. The alloy was then cooled to room temperatureby removal from the furnace. Air had access to the interior of thefurnace.

Bell annealing refers to annealing in whichsheet material was placed ina closed furnace. Air was displaced with an atmosphere resulting fromhydrocarbon combustion. The furnace was brought to a temperature ofabout l,l00 F. over a period of several hours. The furnace was held attemperature for a period of several hours, after which it was slowlycooled over several hours to a temperature at which the furnace could beopened without promoting excessive oxidation.

1n strip annealing, a strip is continuously fed through a furnaceheated, for example, by gas burners or resistance heat and continuouslyexits therefrom with a gas atmosphere in the furnace produced by burningpropane to eliminate oxygen. The rate of travel is adjusted so thatresidence time in the furnace is sufficiently low that the strip reachesthe desired temperature, l,100 F,, in this case.

Temp.Boiling pt. H,SO,a-l2% vol Temp. F.

Time in each30 sec.

Temp-Boiling pt. H,SO, 12% vol Temp. 150 F.

Time in each30 sec.

Temp-Boiling pt. H,SO,-12% vol Temp. I50 F.

Time in each-30 sec.

Treatment Following Bell Anneal NaOH-pH 14 Temp-Boiling pt.

Temp. 150 F.

NaOH-pH 13 Temp-Boiling pt. H,SO a-12% vol Temp. 150 F.

Time in cach-l5 see. Time in eachsec.

NaOH-pH 14 Temp-Boiling pl. H,SO l- 12% vol Temp. l50 F.

Time in each-5 sec.

NaOHpH l4 Temp-Boiling pt. H,SO,12% vol Temp. 150 F.

Time in each-5 sec.

It should be noted that the alkaline immersion for air annealed alloy Cfor short treatment times should use lithium hydroxide. In addition, anacid bath composition modification was used for the bell annealed alloyC treatment. In this latter case, sodium dichromate should be added tothe acid bath in the range of from 1 to 8 ounces per gallon in orderthat the last 25 Angstroms of A1 0 might be removed.

This acid dichromate bath modification can be used with the otheralloys; however, it produces an etched surface which appears hazy orfrosty; an undesirable surface condition for the final product. This isnot important in most instances since further cold rolling and stripannealing will produce a bright surface.

It was found that after the cleaning process described above, all alloyswere successfully soldered after cleaning. By comparison, prior tocleaning, none of these alloys could be soldered.

EXAMPLE ll The efficacy of the foregoing cleaning procedure has beenaccurately demonstrated by utilizing surface capacitive measurementsbefore and after cleaning. It has been found that the cleaning techniquedescribed effectively increases the surface capacitance, a parameterlinked to the thermal oxide thickness and solution double layerthickness, to a value indicative of only the surface double layercapacitance produced by immersion of the test specimen in theelectrolytic solution used in capacity measurement. For example, aspecimen of alloy C before cleaning was covered by a film of oxide whichcapacitance measurements allowed to be calculated 88 Angstroms thick.After cleaning, the thickness of oxide and double layer equivalent was4.8 Angstroms. Correction for the double layer reduces any total valueless than 8 Angstroms to zero thickness of oxide. Effectively, thecapacitance bridge showed that the surface was cleaned. This wasconfirmed by the solder test.

This invention may be embodied in other forms or carried out in otherways without departing from the spirit or essential characteristicsthereof. The present embodiment is therefore to be considered as in allrespects illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims, and all changes which comewithin the meaning and range of equivalence are intended to be embracedtherein.

What is claimed is:

l. A process for cleaning a copper base alloy surface having a thermallyformed oxide thereon, comprising: immersing the surface to be cleaned inan aqueous alkaline solution having a pH of from 1 1 to 14 at atemperature from 160 F. to the boiling point of said aqueous alkalinesolution for a period of time of at least 2 seconds; and then immersingthe surface to be cleaned in an aqueous mineral acid solution at atemperature of from 75 to 200 F. for from 2 seconds to 5 minutes, saidacid solution having an acid equivalent to an H 50, of 3 to 50 percentby volume.

2. A process according to-claim 1 in which the copper base alloy surfacehas a complex oxide thereon.

3. A process according to claim 1 in which the temperature of the acidbath is from to 175 F.

4. A process according to claim 1 in which said alkaline solutioncontains lithium hydroxide.

5. A process according to claim 1 in which said alkaline solutioncontains sodium hydroxide.

6. A process according to claim 1 in which the acid solution consists ofan aqueous solution of H 80 7. A process according to claim 1 in whichsodium dichromate is present in the acid solution in an amount of from 1to 8 ounces per gallon.

8. A process according to claim 1 in which the thickness of the oxideafter cleaning is less than 10 Angstrom units.

9. A process according to claim 1 in which the oxide is at least in parta spinel.

10. A process according to claim 1 wherein the temperature ofsaidalkaline solution is at the boiling point.

11. A process for cleaning a copper base alloy surface having athermally formed oxide thereon, comprising: immersing the surface to becleaned in an aqueous alkaline solution having a pH of from 1 1 to 14 ata temperature from F. to the boiling point of said aqueous alkalinesolution for from 5 seconds to 1 minute; and then immersing the surfaceto be cleaned in an aqueous mineral acid solution at a temperature from125 to F. for a period of from 5 seconds to 1 minute, said acid solutionhaving an acid equivalent to an H 80 solution of5 to 20 percent byvolume.

12. A process according to claim 11 wherein the temperature of saidalkaline solution is at the boiling point. 7

2. A process according to claim 1 in which the copper base alloy surfacehas a complex oxide thereon.
 3. A process according to claim 1 in whichthe temperature of the acid bath is from 125* to 175* F.
 4. A processaccording to claim 1 in which said alkaline solution contains lithiumhydroxide.
 5. A process according to claim 1 in which said alkalinesolution contains sodium hydroxide.
 6. A process according to claim 1 inwhich the acid solution consists of an aqueous solution of H2SO4.
 7. Aprocess according to claim 1 in which sodium dichromate is present inthe acid solution in an amount of from 1 to 8 ounces per gallon.
 8. Aprocess according to claim 1 in which the thickness of the oxide aftercleaning is less than 10 Angstrom units.
 9. A process according to claim1 in which the oxide is at least in part a spinel.
 10. A processaccording to claim 1 wherein the temperature of said alkaline solutionis at the boiling point.
 11. A process for cleaning a copper base alloysurface having a thermally formed oxide thereon, comprising: immersingthe surface to be cleaned in an aqueous alkaline solution having a pH offrom 11 to 14 at a temperature from 160* F. to the boiling point of saidaqueous alkaline solution for from 5 seconds to 1 minute; and thenimmersing the surface to be cleaned in an aqueous mineral acid solutionat a temperature from 125* to 175* F. for a period of from 5 seconds to1 minute, said acid solution having an acid equivalent to an H2SO4solution of 5 to 20 percent by volume.
 12. A process according to claim11 wherein the temperature of said alkaline solution is at the boilingpoint.